During travel of a lift cage in a lift shaft, different forces can act on the cage, which consists of the cage body and a cage frame holding the cage body, and excite the system to vibrate. The causes of vibrations in that case can be, in particular, unevennesses in the guide rails and forces produced by the slipstream about the cage. Beyond that, lateral traction forces transmitted by the traction cables or sudden positional changes of the load during travel can produce transverse vibrations.
In order to increase travel comfort for persons using the lift, regulating systems are employed which are designed for the purpose of providing compensation for forces acting on the lift cage. For example, a system is known from U.S. Pat. No. 5,896,949 which comprises several guide elements connected to the lift cage and movable between two end settings. Vibrations or accelerations arising transversely to the direction of travel are measured by several sensors mounted at the cage and the signals thereof are used for control of a plurality of actuators arranged between the cage and the guide elements. The actuators are in that case controlled by a regulating device, which is connected with the sensors, in such a manner that they work in opposition to the arising vibrations and thereby suppress these as effectively as possible.
A typical characteristic of the method known from U.S. Pat. No. 5,896,949 as well as other methods for reducing vibrations of lift cages in accordance with the state of the art is that these operate with regulators which are linear and invariable with respect to time. The reason for that is that, in the design of the regulator, non-linear processes can be taken into consideration only with difficulty and accordingly for simplification of the concept of the regulator the starting point is that the disturbances which occur are linear. However, the consequence of such an assumption is that undesired vibrations can arise when the regulator is switched on at the beginning and end of travel of the lift. The cause of such vibrations is that non-linear changes in the state of the system are present and cannot be controlled by the linear and time-invariable behaviour of the regulator.